In the field of aeronautics, there is a long standing need for vehicles which can travel with a wide range of controllable speeds. It is desirable to have speed control capability which permits a vehicle to fly at near stall conditions to permit controlled landings and takeoffs. On the other hand, it is desirable to have an ability to fly the same vehicle at speeds in the range of Mach 4 to Mach 7.
It is well known that vehicles can be equipped with rocket engines in order to permit them to fly at speeds above Mach 4. But rocket propulsion can not support a long range sustained flight in the atmosphere because rocket engines has low specific impulse, i.e., low fuel efficiency.
It is also well known that turbojet engines, which use hydrocarbon fuels, have the capability of driving a vehicle with a controllable range of speeds from stall through about Mach 3. An addition of a ramjet can extend this speed range to about Mach 5. But a vehicle with an added ramjet is additionally heavy and complex. Such vehicles have additional drag. Also, integration of two different engine systems presents many operational difficulties.
These and other design dilemmas have led most developers of high speed engines away from hydrocarbon fueled designs. Most recent efforts at developing high speed engines, i.e., above Mach 3, have been directed to turbojet engines which use cryogenic hydrogen as a fuel. Examples of these engines are disclosed in (ATREX Paper from 1995) and U.S. Pat. No. 5,272,870, (Greib, et al.).
This design evolution has occurred largely because of the high stagnation temperatures which occur in front of compressors of these engines at high Mach numbers. Temperatures in the air inlet of a vehicle traveling at Mach numbers in the range of 5 to 6 reach 1300.degree. to 1700.degree. K
As a result there is a recognized need to perform pre-cooling of incoming air before it reaches the compressor. This prior art recognition of a need for pre-cooling has been met with heat exchangers that utilize a cooling effect of liquid hydrogen expanding into a gas. In other words, the prior art cryogenic engines use cooling properties of their fuels as a mechanism for pre-cooling of incoming air.
While these cryogenic systems can produce the needed cooling, they do so at a high cost in vehicle weight and complexity. Closed system heat exchangers are inherently heavy and they produce undesirable drag.
Additionally, use of hydrogen as a fuel has several disadvantages. Hydrogen's low density requires larger vehicle structures. Cryogenic hydrogen is difficult to handle. Difficulties in handling hydrogen are encountered at sea level where fueling of aircraft occurs. The difficulties do not end on the ground. Safe handling of hydrogen in flight requires many expensive and complex control systems on a vehicle. As a result, vehicles with hydrogen engines become very expensive.
Thus, when the complexity of hydrogen cooling is added to the inherent complexity of hydrogen fuel handling, the resultant engine emerges as one which is realistically applicable only to a narrow class of vehicle, namely Earth-to-orbit launchers. There is a clear need for a simpler design. This need for a simpler engine design is particularly acute for propelling expendable low-cost vehicles.
It is a goal of the present invention therefore to provide an aircraft engine which can produce a wide range of aircraft speeds and which is based on relatively simple hydrocarbon fuel technology.